It varies a lot through canons, and I'm trying to solidify my own as well as look at other views.

My current Headcanons:

-Administrator is a neutral party, pretty much retired now.
-05s are, well, 05s.
-Directors receive orders from 05s, and are Level 4 themselves, if not with more administrative duties. 1-2 per Site. Delegate to Level 4s 'technically' below them.
-Level 4s each manage one department. 4-5 per site. It's here where the team inevitably gets more tightly knit as time goes on despite attempts not to; these are people who work on the same site together for 15, 20, 30 years without changing positions. They become more like family, in a weird, convoluted, panicky way.
-Level 4s handle day to day workings of the site on a department basis. Designate Site-wide announcements, Department-wide announcements, and to level 3s, which largely carry out experiments on a daily basis under Level 4 supervision.
-etc

The O5s, 13 in number, live and work at a number of highly classified Protected Sites around the world, most prominent of which is Protected Site 1. The O5s (at least publicly) try to interact with anomalies as little as possible, and thus rarely leave their designated Sites. Instead, they hold meetings with Level 4 personnel, which involves Site Directors, Mobile Task Force and Tactical Response Force commanders and Ethics Committee members. The exception to this is O5-13, whose position is O5 clearance, and who acts as a tiebreaker as well as handles the Thaumiel and SCP-001 stuff.The Ethics Committee is led by several O5 members to ensure that a single member does not gain too much power. Site Directors are Foundation personnel with Level 4 clearance who govern individual sites and commune with the Site's Head Researchers, who have Level 3 Clearance and oversee the research of individual SCPs and the personnel working on them. There are also other Foundation employees who require Level 3 or 4 clearance for their work but do not directly interact with SCPs, such as Personnel Directors or Containment Specialists. The majority of the Foundation has Level 2 clearance. This involves Researchers, who are the people who do the actual testing, and Agents, who man Observation posts and are in charge of finding and gathering evidence for SCP objects as well as gathering information about GoIs and sometimes recovering Safe- and Euclid-class objects, and who may work undercover as local law enforcement, scientists in non-anomaly fields, or GoI members. Other personnel with Level 2 clearance include Security Officers, who are in charge of keeping the SCPs inside and the GoI members outside. Mobile Task Forces are teams who are in charge of recovering or recontaining SCPs, and who often move between Sites, while Tactical Response Forces are stationed at a single site and are in charge of recontaining SCPs that have breached containment and defending the Site from hostile groups. Members of MTFs or TRFs Have Level 2 or 3 clearance, while the commanders of such teamms havve Level 4 clearance.

My canon largely aligns with Feline's, with the biggest exception being O5's running the Ethics Committee. For me, Ethics is independent of, and superior to, the O5 Council. Ethics Committee personnel, when they are acting on behalf of the committee, have clearance even beyond O5, partially because nothing can be redacted or expunged from them (or so help you God), and partially because it doesn't matter as much if their individual members get exposed to anomalies. The Foundation doesn't grind to a halt if one gets zapped (although somebody's going to answer for it.)

Blanket level 4 clearance is extremely limited: site directors, some MTF leaders, some high level agents or administrative personnel, and lead researchers on very difficult Keters. (Researchers important enough to hit level 4 usually become site directors.) Most lead researchers on specific SCPs in the Foundation are level 3 or level 4/####. Similarly, there are a fair number of agents with 4/CODEWORD clearance. Generally speaking, 4/<something> is way more common than 4. In much the same way, there are lots of level 2's with 3/#### clearance on their specific SCPs/jobs. I agree that the vast majority of active personnel (e.g., not front company/contractors) are level 2, or level 3 with seniority.

There's also another factor to this, which we've worked around a bit, but clearance and rank/seniority/role in the chain of command aren't entirely synonymous. It's generally true that any career move in the Foundation that increases your stature/responsibilities also increases your access to secrets (i.e., clearance), but that doesn't mean you can walk into a room shouting "I'M A LEVEL 4, EVACUATE THE SITE!" That might work with level 2 newbies who don't know any better, but it's a damn good way to get decked by an MTF agent.

That might work with level 2 newbies who don't know any better, but it's a damn good way to get decked by an MTF agent.

^ /thumbsup

And also the whole "just because you're Level-4, doesn't mean you get to carry a machine gun everywhere bcuz raisons and scare the crap out of regular research staff". Low-quality writing aside, it befuddles the reader to hear about a scientist carrying weapons that they don't need to do their science job, and are very likely not even trained to use. Job descriptions! Things! Excitement!